Many hydrocarbon gas streams, in addition to the predominant methane component, contain varying amounts of heavier hydrocarbons such as ethane, propane, butane, etc., as well as impurities such as acid gases which are typically carbon dioxide and/or hydrogen sulfide. It is often necessary to process such hydrocarbon streams to remove the impurities and to separate the heavier hydrocarbon components which are also valuable and quite often have different end uses than methane. Thus, for example, in order for natural gas to be commercially acceptable, it must meet stringent specifications with respect to heating value and hydrogen sulfide and carbon dioxide contents. Consequently, sufficient hydrogen sulfide must be removed so that the natural gas has a hydrogen sulfide concentration of no more than about one quarter to about one half grain per 100 standard cubic feet. By the same token, the carbon dioxide content should be less than about 2 mole percent, since higher concentrations can be corrosive and may reduce the heating value of the natural gas to an unacceptable level.
Removal of acid gases from hydrocarbons may be undertaken by use of a number of established technologies. Thus, for example, it is known to use physical solvents which are selective toward the acid gas components and chemical solvents which will react with such components. Examples of appropriate physical solvents include propylene carbonate and the dimethyl ether of polyethyleneglycol. Examples of suitable chemical solvents are aqueous solutions of potassium carbonate and of amines such as monoethanolamine, diethanolamine, etc. More recently, it has been proposed to use semipermeable membranes, as in U.S. Pat. No. 4,130,403. However, it is not considered economical to produce large membrane elements or units, and consequently, membrane systems do not enjoy the same economy of scale that conventional processing enjoys and their use for large scale applications has been limited.
There are also several methods which are known to remove hydrocarbon components heavier than methane. In some instances, merely cooling the hydrocarbon stream will condense part of the heavier components to liquids which may then be separated from the uncondensed portion and further separated into the individual components, e.g., ethane, propane, butane, etc. Another method for recovery of such hydrocarbon liquids is by absorption in a hydrocarbon oil. In this method, ethane and the other heavier components are dissolved in oil in an absorber. The oil containing the dissolved components then flows to a stripper in which the hydrocarbon components are desorbed by the application of heat.
The most recently developed technology for separating and recovering the hydrocarbon liquids is carried out at cryogenic temperatures in which the refrigeration may be supplied, at least in part, by expanding the gas while performing work in a device called a turboexpander. The condensed liquids may then be separated by low temperature distillation.
If the hydrocarbon stream also contains carbon dioxide and/or hydrogen sulfide, such components are usually removed prior to separation of the hydrocarbon liquids. In the case of separation at cryogenic temperatures, the acceptable and preferred practice is to remove the carbon dioxide and any water vapor that may be present prior to cooling, since both water and carbon dioxide can become solid at low temperature and thereby plug the equipment. Under certain conditions, however, carbon dioxide will remain in the liquid state, and its separation by distillation may be preferrable to other methods. Thus, in U.S. Pat. No. 3,595,782, a process is described in which water is removed before the gas stream encounters cryogenic temperatures, but the carbon dioxide is separated from the condensed liquid by distillation at low temperature. In such process, the carbon dioxide is removed overhead along with methane, while ethane and heavier hydrocarbon components are removed as a bottom product of the distillation. This process has a disadvantage, however, in that the carbon dioxide remains with the methane and must ultimately be separated therefrom, unless the carbon dioxide content is relatively low. Also, if hydrogen sulfide is not removed before the gas is cooled, it will be separated as a liquid along with the heavier hydrocarbon liquids, and if the individual hydrocarbon components are subsequently separated, the hydrogen sulfide will appear with the hydrocarbons, primarily the propane and ethane, and must then be removed therefrom.